Compared to the Thin Film Transistor Liquid Crystal Display (TFT-LCD) which is the main display technology at the present, the Organic Light Emitting Diode (OLED) display now has become the focus of concern due to its advantages of wide view angle, high brightness, high contrast, low power consumption, small thickness and volume, and the like.
The driving methods for the OLED display may be classified into two types, i.e., Passive Matrix (PM) driving and Active Matrix (AM) driving. Compared to PM driving, AM driving has advantages of large amount of display information, low power consumption, long service life of devices, high contrast of images, and so on. In an AMOLED display in the prior art, as shown in FIG. 1, the equivalent circuit of the basic principle of the pixel unit driving circuit comprises a switching transistor M1, a driving transistor M2, a storage capacitor C1, and a light emitting device D1, wherein the drain of the switching transistor M1 is connected with the gate of the driving transistor M2, the gate of the driving transistor M2 is also connected with one end of the storage capacitor C1, the source of the driving transistor M2 is connected with the other end of the storage capacitor C1, and the drain of the driving transistor M2 is connected with the light emitting device D1. The switching transistor M1 is switched ON when its gate is gated by the scanning signal Vscan(n) so that the data signal Vdata is introduced therein from its source. The driving transistor M2 typically works in the saturation region, and its gate-source voltage Vgs determines the amount of the current flowing through the driving transistor M2 so as to apply stable current to the light emitting device D1, wherein Vgs=Vdata−VD1, and VD1 is the voltage drop across the light emitting device D1 when the light emitting device D1 has a luminance of the highest gray scale. VDD is a stabilized voltage or a stabilized current supply, is connected to the source of the driving transistor M2, and supplies the energy needed for causing the light emitting device D1 to emit light. The storage capacitor C1 serves to stabilize the gate voltage of the driving transistor M2 during one frame period.
Typically, a pixel unit driving circuit of the OLED display comprises sub-pixels of three different colors of Red (R), Green (G) and Blue (B). As shown in FIG. 2, it is easily seen that: although the light emitting layers of the sub-pixels of different colors are fabricated with different materials, the pixel unit driving circuit of the OLED display drives the red organic light emitting device DR, the blue organic light emitting device DB and the green organic light emitting device DG in the pixel unit by the driving voltage VDD of the same driving power (the voltage of the driving power is a driving voltage necessary for the largest luminance of the blue organic light emitting device DB). However, since the light emitting layers in the organic light emitting devices of the three different colors have different semiconductor materials, the voltage drops across the organic light emitting devices of the three different colors are different from each other. It can be seen from FIG. 3 that, when the sub-pixels of three colors have identical display luminance, the voltage drop across the blue organic light emitting device DB is larger than the voltage drop across the red organic light emitting device DR, and the voltage drop across the red organic light emitting device DR is larger than the voltage drop across the green organic light emitting device DG. Because the pixel unit driving circuit of the OLED display drives the organic light emitting devices of three different colors in the display by the same driving power voltage, the difference between the actual voltage drop across the driving transistor TG of the green sub-pixel and the voltage drop necessary for itself is the largest, the difference between the actual voltage drop across the driving transistor TR of the red sub-pixel and the voltage drop necessary for itself is the second largest, and the difference between the actual voltage drop across the driving transistor TB of the blue sub-pixel and the voltage drop necessary for itself is the smallest. Thus, the power consumption on the driving transistor TG of the green sub-pixel is too large, which results in resource waste. Furthermore, since the bias voltages of the driving transistors in the sub-pixels of the three different colors are different from each other, the driving transistors cannot work with their best driving ability.